kuniです。 みなさん、アドバイスありがとうございます。
/var/lib/mysql/*.err に原因が記録されていませんか?
errログのファイルを見てみました。 以下upgrade時の不具合発生時点と思われる部分のログを引用してみます。 061128 19:40:44 mysqld started InnoDB: Resetting space id's in the doublewrite buffer 061128 19:40:44 InnoDB: Started; log sequence number 0 43912 InnoDB: You are upgrading to an InnoDB version which allows multiple InnoDB: tablespaces. Wait that purge and insert buffer merge run to InnoDB: completion... InnoDB: Full purge and insert buffer merge completed. InnoDB: You have now successfully upgraded to the multiple tablespaces InnoDB: format. You should NOT DOWNGRADE to an earlier version of InnoDB: InnoDB! But if you absolutely need to downgrade, see InnoDB: http://dev.mysql.com/doc/refman/5.0/en/multiple-tablespaces.html InnoDB: for instructions. 061128 19:40:46 [ERROR] bdb: unable to initialize mutex: Function not implemented 061128 19:40:46 [ERROR] bdb: process-private: unable to initialize environment lock: Functio n not implemented 061128 19:40:46 [Warning] './mysql/host' had no or invalid character set, and default charact er set is multi-byte, so character column sizes may have changed 061128 19:40:46 [Warning] './mysql/user' had no or invalid character set, and default charact er set is multi-byte, so character column sizes may have changed 061128 19:40:46 [Warning] './mysql/db' had no or invalid character set, and default character set is multi-byte, so character column sizes may have changed 061128 19:40:46 [ERROR] Fatal error: mysql.user table is damaged or in unsupported 3.20 forma t. 061128 19:40:46 mysqld ended 復旧手順の確認ですが、 1.MySQL （php-mysql も）を全て apt-get remove これを実行した場合、DBも一緒に削除されないでしょうか？